Minor header fixes for Typography Refresh

This fixes top-margin for H1 inside .mw-body-content, and increase
font-size for H3 from 1.17em to 1.2em (bumping to 17px default).

Final patch for this bug.

Bug: T66653
Change-Id: I1e75bc4fc3e04ca6c9238d4ce116136e9bafacd1